Q&A: Venting a Shed Dormer

Forced Air vs. Hydronic Heat

Q.We build a lot of shed
dormers that start at the ridge and have a roof pitch
of about 3:12. I have never installed a ridge vent
because I am concerned that snow will build up and leak
through the vent on the dormer side. Now the code
inspector insists the roofs must be ventilated. How
should we do this?

A. A ridge vent should work at
a pitch of 3/12, but manufacturers warn against
using one at any slope less than that. The vent
should have a snow baffle to keep snow from blowing
up the dormer roof and into the vent, and the snow
baffle must have weep holes so that melting snow
can drain from it. When the wind is blowing from
the other (nondormered) side of the roof, air will
be blowing out the lee side vent, keeping it clear
of snow.

When the slope is less than 3/12, recent
research indicates that no venting should be used
(though convincing the code official of this may
not be so easy). Research by William Rose at the
University of Illinois Small Homes Council-Building
Research Council indicates that if a vapor retarder
is installed in the ceiling and the rafter space is
sealed tightly and stuffed with insulation,
ventilation is not required. The rafter spaces
should be essentially airtight, and have absolutely
no penetrations of the ceiling from inside the
house.